In recent years, in application to an optical material of a micro lens for a digital camera module, a pick-up lens, an imaging lens, an optical element (such as a micro lens array, an optical waveguide, an optical switching, a Fresnel zone plate, a binary optical element and a blaze diffractive optical element) and the like, and in application to an electronic device material of an anti-reflective filter, a recording medium, a display material, an organic EL or liquid crystal plastic member and the like, a cyclic olefin polymer having excellent optical properties has been studied for development of wide application in these fields.
A polymer material containing a cyclic olefin polymer is an aggregate having a long molecular chain and a polymer molecular chain itself has anisotropy that is an inherent physical property, so that it is melt-flowed by the pressure or temperature at the time of melt molding and is cured in the form of a molded product on a mold or a roll in a state that flowing of a resin is not fully reduced in many cases. In this case, there has been well known that a chain having a long molecular chain exhibits the orientation of the molecule under molding conditions.
The orientation of the molecule is attributable to the anisotropy that is an inherent physical property of the molecular chain itself, and optically produces the anisotropy of the refractive index. When the birefringence is produced as the effect of anisotropy of the refractive index on a molding material and a birefringence material is present in the optical path, for example, image quality of the product, readout of the signal or the like is adversely affected. In development of an optical material and an electronic device material consisting of an optical resin, reduction of the birefringence is an important issue.
Herein, in the appropriate technical field, as the birefringence exhibited by an optical polymer, there are the orientation birefringence in which its main cause is derived from the main chain of a polymer and the stress birefringence caused by the stress. Furthermore, each sign of the orientation birefringence and stress birefringence is derived from the chemical structure of a polymer, and is a property inherent in each polymer.
In an attempt to reduce the orientation birefringence or stress birefringence, a method of cancelling the birefringence has been attempted with the addition of an inorganic fine particle or an organic compound having birefringence properties of an opposite sign to a polymer. When an organic compound is added, for a film with the addition of 6.5 wt % 2-octadecyl naphthalene exhibiting positive birefringence properties to polymethyl methacrylate exhibiting negative birefringence properties, the orientation birefringence becomes almost zero regardless of the stretch ratio (for example, Patent Document 1). Furthermore, for a stretched film consisting of a copolymer of methyl methacrylate and benzyl methacrylate with the addition of a strontium carbonate fine particle having a particle diameter of about 20 nm as the inorganic fine particle in an amount of 0.3 wt %, the orientation birefringence also becomes almost zero regardless of the stretch ratio (for example, Non-patent Document 1). These methods are excellent, but there is a practical problem such that it is necessary to add an additive material continuously and uniformly when adapted to opacification of a film due to aggregation of an additive material, size control of fine particles or the process.
Meanwhile, there has been exemplified a method in which, in a non-additive material without the addition of an organic compound, an inorganic fine particle or the like to a polymer, any one of the orientation birefringence or stress birefringence is reduced, or both of them are reduced by copolymerizing two or more kinds of methacrylates or acrylate monomers having birefringence properties with a different sign of birefringence, and changing the kind and copolymerization composition ratio thereof (for example, Patent Document 2 and Non-patent Document 2). These methods are excellent since an additive material is not used and there is no need to mix an additive material continuously during process without causing opacification of a film due to aggregation.
In this example, for example, in the case of a copolymer of methyl methacrylate and pentafluorobenzyl methacrylate of a certain composition, both of the orientation birefringence and stress birefringence are reduced to a level that there is no practical problem. However, generally, to obtain a film by subjecting a methacrylate polymer to melt molding, the polymer is heated up to a temperature that can perform melt molding and a resin stays. In this case, there is a problem such that the gel component derived from monomers or oligomers caused by depolymerization is mixed into a film and film performance is thus deteriorated. Also, the glass transition temperature of the methacrylate polymer is low, that is, about 100 degrees centigrade, so that heat resistance becomes a problem depending on the purposes when the polymer is used for a product. There are some polymers imparted with heat resistance by increasing the glass transition temperature, but in this case, it is necessary to set the melting temperature to a high level during molding for increased glass transition temperature, and mixing of a gel-like material into a film is possibly more remarkable due to the aforementioned depolymerization.
On the other hand, a cyclic olefin polymer is a polymer which is amorphous and transparent, and has a relatively high glass transition temperature by a rigid cyclic structure in the main chain, and it is widely used for an optical film, an optical lens or the like using such excellent properties. Also, in an attempt to reduce the birefringence, the photoelastic constant is exemplified as a measure to exhibit the stress birefringence, for example, for a hydrogenated product of ring-opening polymerization of a norbornene or tetracyclododecene monomer (Non-patent Document 3). In this case, for a hydrogenated product of norbornene ring-opening polymerization containing an alkyl group, a phenyl group or the like in which a substituent has a relatively low polarity, the photoelastic constant is relatively high, that is, about 10 to 30×10−12 Pa−1.
With respect to the orientation birefringence of a cyclic olefin polymer, there is exemplified the phase difference of a stretched film of an amorphous cyclic olefin copolymer which is obtained by addition polymerization of a cyclic olefin having a norbornene basic structure, particularly preferably a cyclic olefin having a hydrocarbon structure derived from norbornene or tetracyclododecene or derived from them, and a non-cyclic olefin having a terminal double bond, for example, α-olefin, particularly preferably ethylene or propylene, among these, particularly preferably norbornene and ethylene, norbornene and propylene, tetracyclododecene and ethylene, and tetracyclododecene and propylene (Patent Document 3). In this evaluation, for a film having a thickness of 40 to 60 μm which is stretched by 1.5 times to 4 times, the phase difference is from 5 to 16 nm, and 1 to 3×10−4 in terms of a converted value of the orientation birefringence. By an attempt to reduce the birefringence, relatively low birefringence is exhibited. However, from the fact that the copolymer in Patent Document 3 is a copolymer of a cyclic olefin and a molecular chain of an α-olefin chain in the main chain without a molecular design to cancel the birefringence by the anisotropy of the molecular chain among a plurality of cyclic olefin species, toughness is poor. For example, when a highly rigid substrate such as glass is used as a base in the production of a cast film, in a process of drying a film, the film is broken due to shrinkage stress of the film during cooling after heating. Or, even though a self-support film having a resin material as a substrate is produced, it is easily broken due to external stress and thus it is not easy to obtain a film having a large area.